Tiers indicate what games mean to me. Rating is the actual quality of a game (given its context and time period).
Mmmhhh... these are all the games I remember playing, and for long enough to form an honest opinion on them. There might be a handful missing (especially from longer ago), and I will add them when I think of them. But 90%+ of all games I've ever (seriously) played should be on here.
Extremely stable! Let me explain.
The tier a game belongs to is how 'impactful' it was to me, how much I enjoyed it given its time and context. There might then also be several cases where a high tier does not automatically imply a recommendation.
New games added (as I play them) will be inserted into the list. But this will leave the list otherwise stable.
These indicate how high I rate the quality of the game (also given its time period). There's a strong correlation between high tiers and high ratings.
Notable exceptions include for example games that mean a lot to me, but might still have a rather low rating because they're still bad games. Vice versa some games with high quality didn't click for me, so they might end up in a low tier.